Design, processing and characterization of advanced titanium scaffolds with controlled radial porosity: a new sequential compaction device

摘要

Introduction: Biological systems like those that are involved in many human tissues are perhaps one of the most exciting and inspiring examples of graded highly anisotropic hierarchical materials. In this work, we are initially focused on phenomenological relationships between bone structure, its mechanical properties and most frequent damages. Current status of biomedical materials for bone replacement clearly indicates that titanium (Ti) and some of its alloys are still the better metallic material clinically used to that purpose. The stiffness mismatch between Ti and bone, around one-order magnitude (～110 GPa against ～20GPa), is one of those current challenges that require to be overcome; the associated stress shielding is main the cause of the clinically observed bone resorption around Ti implants. This is an ideal scenario in which functionally graded materials (FGMs) become a powerful alternative to offer new alternatives for better bone implants. The aim of this work is the development of a novel compaction device that responds to necessity of fabricating new components with controlled graded radial porosity. Materials and Methods: Sequential compaction device. The novel sequential compaction device developed by the authors and here used in is suitable for both conventional PM and space-holder methods, as well as their possible combinations. It is also appropriate to form layered structures of any material or combinations of them, than can be obtained by powder technologies. Specifically, this device allows obtaining parts with controlled, discrete, increasing, decreasing or alternative radial porosity, ensuring the structural integrity of the part. Porous Ti cylinders, as candidates for bone tissue replacement were fabricated from powder of commercial pure titanium (Ti cp, grade Ⅳ, ASTM F67-00); sodium chloride (NaCl, purity ＞99,5%) was used as space-holder. Ti powder was supplied by SE-JONG materials company Ltd (Republic of Korea), and NaCl from Panreac Quimica S.A.U. (Spain). Ti powder exhibited an irregular morphology, whilst NaCl showed regular cubic morphology. Results and Discussion: The main goal of this work was to evaluate the efficiency of the new compaction device proposed to develop cpTi specimens with a controlled radial and graded porosity; this evaluation is carried out in terms of process repeatability, ability to replicate porosity parameters previously designed, cost, structural integrity, and interfaces adhesion. As can be observed in Fig. 2, green body of cpTi has a high structural integrity, before NaCl removal. Additionally, it is clearly detected those three zones with the increasing radial graded porosity, from core to inner and outer shells. The implementation of this sequential compaction device implied optimizing of pressing and extracting processes: proper conditions were 10N/s and 30N/s for cpTi cylinder. Conclusions: A novel sequential compaction device has been successfully designed and fabricated, which has shown to be able produce FGM porous cylinders with controlled radial graded porosity. Specifically in this work, cpTi were designed and fabricated by using this unique device, which is actually within a patent process. The whole characteristics of porous samples produced with this compaction device clearly indicate that process is repeatable, economic and versatile.